To improve light-emitting efficiency of the GaN-based LED, the substrate transfer technology has been developed in recent years. The transfer processes may include: depositing a GaN-based thin film over a sapphire substrate via MOCVD, bonding the GaN-based thin film to a semiconductor or metal base using wafer bonding technology or electroplating technique, and removing the sapphire substrate using laser lift-off method; or depositing a GaN-based thin film over a SiC or Si substrate, bonding the GaN-based thin film to a semiconductor or metal base using wafer bonding technology or electroplating technique, and removing the SiC or Si substrate using chemical etching method. In this way, the thin-film GaN chip can achieve a higher light-emitting efficiency either by adding a reflecting layer between the epitaxial thin film and the base or by easily obtaining a rough light-emitting surface using photochemical etching method over the N-polar surface GaN. At the same time, the GaN-based thin-film chip transferred to a heat dissipation base has relatively large advantages in large-current applications due to good thermal conductivity of the transferred base.
However, the surface of the exposed GaN-based thin film after removal of the growth substrate is generally an N-polar surface. The ohmic contact property of the N-polar surface is different from that of the Ga-polar surface. For example, the N-type GaN ohmic contact electrode with a Ga-polar surface, in general, adopts a Ti/Al ohmic contact electrode. If the N-type GaN contact electrode with an N-polar surface adopts a Ti/Al ohmic contact electrode, at the initial time, the Ti/Al demonstrates a better ohmic contact property with the N-type GaN than with the Ga-polar surface. However, when the temperature rises to about 150° C., the contact property will degrade to Schottky contact and show a rise of forward working voltage, which seriously restricts the light-emitting efficiency of the thin-film GaN chip. In accordance with some representative discussions about the cause: Hyunsoo Kim et al. (APPLIED PHYSICS LETTERS 93, 192106, 2008) believed that it is attributed to the N vacancy and surface Ga vacancy as well as the atomic reaction of C and O, which lead to reduction of surface N vacancy; Ho Won Jang et al. (APPLIED PHYSICS LETTERS 94, 182108, 2009) reported that it is the diffusion of intrinsic N atom to the surface that leads to the reduction of surface N vacancy. By now, neither of the two study groups has put forward effective method for fabricating an N-type GaN ohmic contact electrode over the N-polar surface. In the thin-film flip-chip (TFFC) LED introduced by Philips Lumileds Lighting Company, the N-type ohmic contact electrode is still fabricated over the Ga-polar surface N-type GaN, i.e., it can use the Ti/Al ohmic contact electrode. Therefore, one notable advantage of TFFC is that it can totally avoid the N-polar surface problems as aforementioned. However, it exerts a higher requirement for chip inversion technology since the P and N electrodes over the thin film are required to be bonded to the positive and negative electrode areas of the base respectively. In addition, to prevent the thin film from breaking during laser lift-off of the sapphire substrate, the thin film surface, at the instant of laser lift-off of the sapphire, should be ensured to bear uniform impact force. Therefore, a medium should be filled between the thin film and the inverted bonding base before the laser lift-off, which may influence yield of device since it is difficult to control the filling consistency.